The correct option is A 0.1 M CH3COOH+0.1 M CH3COONa
Buffer mixture: Mixture of weak acid and its salt with strong base can act as an acidic buffer.
1. CH3COOH is a weak acid. CH3COONa is a salt of CH3COOH (weak acid) and NaOH (strong base). So the resultant 0.1 M solution is called a buffer solution. So it is correct.
2. 0.1 M NaCl is a mixture salt of 0.1 M NaOH (strong base) and 0.1 M HCl(strong acid). Therefore it is not a buffer solution.
3. 0.1 M CH3COONa is a salt of weak acid 0.1 M CH3COOH and strong base 0.1 M NaOH . It will be a salt hydrolysis reaction, not a buffer solution.
4. 0.5 M HCl is a strong acid and NaCl is a salt of strong acid (HCl) and strong base (NaOH). So it is not a buffer solution.